Field of the Invention
The present invention relates to a quantum cascade laser structure having engineered interface-roughness scattering in the active region to achieve simultaneous increase of the lifetime of the upper laser state and decrease of the lifetime of the lower laser state to significantly reduce laser threshold. The invention furthermore relates to a quantum cascade laser (QCL) having significantly reduced laser threshold based on engineered interface-roughness scattering in the active region.
The quantum cascade laser (QCL) was first demonstrated in 1994 and described as a stack of active regions and digitally graded alloys (injector regions) made out of InGaAs quantum wells and InAlAs barriers lattice matched to InP (Faist et al., Science 264, 553, 1994). Population inversion was achieved by reducing the spatial overlap of the upper and lower laser states and the resonant longitudinal-optical (LO)-phonon depopulation of the lower laser state into the underlying confined states (Faist et al., Science 264, 553, 1994). Since the invention of the QCL, a number of design variations have been proposed, each improving the QCL performance in different respects. Some of the designs, like superlattice active-region, bound-to-continuum, and injectorless designs, differ significantly from the original QCL description (Faist et al., Science 264, 553, 1994), both in the layer structure, and in the way the population inversion is achieved. Nevertheless, all the above approaches are referred to as QCLs, as the name expresses the essential quantum nature of the exploited transitions and the unique possibility to stack (cascade) several emitting periods together.
The initial understanding of the scattering processes in QCL was focused on inelastic LO-phonon scattering, believed to be dominant. Indeed, the LO-phonon scattering usually dominates in quantum wells at room temperature and above, but elastic scattering generally dominates at cryogenic temperatures. The impact of the interface scattering rate depends both on the barrier height and on the interface roughness. Thus, for systems with smooth interfaces and low barrier height, such as AlGaAs/GaAs, LO-phonon scattering is expected to dominate and has been demonstrated to do so through magnetic field measurements. The same experiment, however, carried out on strain-compensated high-barrier InP-based QCL shows elastic scattering (Semtsiv et al., Appl. Phys. Lett. 93, 2008).
Altogether, the impact of elastic scattering is high, even at room temperature, particularly for materials with high-barriers and rough interfaces. At the same time, high-barriers based on strained heterostructures are advantageous for short wavelength QCLs because they provide the necessary confinement of the upper laser state. The high internal strain precludes the possibility to smooth the interfaces by growth interruptions.